OK. I've now been to the Dali Theatre and Museum in Figueres. I was the first person in the door when the place opened at 9 AM today. It was raining and a chilly 59F, which may have reduced the number of people who showed up early to buy walk-up tickets.
I suggest asking for a leaflet with a diagram of the museum so you don't miss anything, which would otherwise be easy to do. I didn't see such a leaflet, but I later observed members of a tour group with something in hand. I assume it had a diagram. The rooms are numbered (up to about 22), so in a pinch one could keep a list.
I had a few uncrowded minutes nearly alone on the ground floor before more people arrived, including at least one tour group. The ground floor was busier than the (US) second floor, but it really dropped off after that. By about 9:15 it was necessary to weave through clumps of people, especially on the ground floor. However, this was more an issue of taking a bit of extra time than of difficult access to the art. As usual, there were quite a number of people just standing around, taking up space, rather than looking at anything.
I would certainly recommend a 9 AM ticket if you can manage it, but at the moment conditions don't seem too bad. (However, this is by comparison to the worst of the sights in Barcelona.) Obviously, I have no idea how much worse the museum gets. There were a lot of French-speakers, but are they going to be spending part of their summer vacations in Figueres? I do not know.
I spent about 1 hr. 45 min. in the museum proper with no picture-taking. I don't think I missed anything, but the layout is convoluted. There's not a tremendous amount of explanatory material (what there is, is mostly translated into English), which made the visit faster for me than it would otherwise have been. It is not a small place at all, and I don't think anyone interested in Dali's work (which I really am not, particularly) would be satisfied with less time than I had in the museum--though much of what is on the top two floors is not by Dali himself--and I doubt very much that any mass-market tour also going to Girona and Besalu stops there for more than an hour.
In addition to the art museum (which has some jewelry and metalwork), there's a separate exhibition of Dali's Jewels in a different building very nearby. You will see the sign on the corner when you make the final turn toward the main museum. The Theatre and Museum ticket covers the jewels exhibition as well. I don't think buying a ticket just for the jewels is possible. I spent 30 minutes in the jewels exhibition because I took a lot of pictures. I got a feeling we were routed away from at least one additional room, so I think it's possible the visit could be longer.
I asked about whether one could visit the jewels before the art museum and was told one could. However, not having done it myself, I can give no guarantees. I don't know whether the jewels exhihition opens at 9 AM or later.